I saw this plant all over the place in Vermont while I was hiking the Appalachian Trail last summer. I also saw it to a lesser extent in Massachusetts and New Hampshire. It was anywhere from one to eight feet tall, but usually about 3 feet tall. The roots of several plants were always connected. (Maybe they were all the same plant.) The root system acting like a vining plant, but I never saw any tendrils on part of the above ground plant. But like I said, it was all over the place, thickly covering the ground in any given area. There were usually other plants growing near it though, so I don't think it is an invasive species that crowds out native plants. I've scoured the internet for hours and have come up with nothing. The closest I saw was either American or Asiatic Bittersweet, but the berry is definitely not the same. Thank you for your help.